An imaging device that irradiates an object with light emitted from a quantum cascade laser, feeds light reflected from the object back to the quantum cascade laser, and measures a change in the electrical characteristics of the quantum cascade laser to acquire the imaging information of the object is known (see Paul Dean, et al., “Terahertz imaging through self-mixing in a quantum cascade laser”, OPTICS LETTERS, Optical Society of America, Jul. 1, 2011, Vol. 36, No. 13, p. 2587-2589).